Support planar and tapered quasi-planar germanium waveguides for infrared evanescent-wave sensing

ABSTRACT

Miniature planar IR waveguides of thickness 30–50 μm, consisting of 12-mm long, 2-mm wide strips of Ge supported on ZnS substrates and tapered quasi-tapered waveguides, tapered from a thickness of 1 mm at the ends to a minimum of 1–100 μm at the center, are disclosed. The surface sensitivity is increased as a function of incidence or bevel angle. The tapered waveguide improves the efficiency of the optical coupling both into the waveguide from an FTIR spectrometer, and out of the waveguide onto a small-area IR detector. The tapering makes it possible to dispense with using an IR microscope couple light through the waveguide, enabling efficient coupling with a detector directly coupled to an immersion lens. This optical arrangement makes such thin supported waveguides more useful as sensors, because they can be made quite long (e.g. 50 mm) and mounted horizontally. Furthermore, even with a 20-μm×1-mm cross section, sufficient throughput is obtained to give signal/noise ratios in excess of 1000 over most of the 1000–5000 cm- 1  range, with just 2 min of scanning at 8 cm- 1  resolution.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of Ser. No. 09/426,055, nowU.S. Pat. No. 6,496,636, filed Oct. 25, 1999 and issued on Dec. 17, 2002which is a continuation-in-part of Ser. No. 08/874,711, now U.S. Pat.No. 5,980,831, filed Jun. 13, 1997 and issued Jun. 13, 1997. The presentapplication further claims the benefits under 35 U.S.C. 119(e) ofcopending provisional patent application Ser. No. 60/106,132, filed Oct.29, 1998. This application incorporates by reference, as though recitedin full, the disclosure of parent U.S. Pat. Nos. 6,496,636 and 5,980,831and copending provisional patent application Ser. No. 60/106,132.

GOVERNMENT GRANTS

This work was supported by NSF grants MCB-9722887 and MCB-9406681 toMark S. Braiman.

BACKGROUND OF THE INVENTION

The development of mid-infrared (IR) waveguides has been driven by theiruse as remote or small-sample-size chemical sensors for surfacesensitive spectroscopy. Such waveguides can be thought of asminiaturized multiple reflection elements (MREs) wherein the incidentlight undergoes total internal reflection at the interface between mediaof different refractive indices. At each internal reflection within thewaveguide, a portion of the optical field, the evanescent wave, extendsbeyond the high-index waveguide into the adjacent low-index medium, to adepth (d_(p)) dependent on the angle of

-   -   incidence and the ratio of the two refractive indices. The        ability of molecules outside the high-index waveguide, but near        its surface, to absorb energy travelling through the waveguide        via this evanescent wave makes possible the phenomenon known as        attenuated total reflection (ATR) or evanescent-wave        spectroscopy (EWS).

In the IR region, high-refractive-index materials as Ge, Si, and KRS-5(Tl₂BrI), cut and polished as prisms having trapezoidal or parallelogramcross-sections and dimensions on the order of 50×20×2 mm, are in commonuse for EWS measurements. These macroscopic waveguides typically havethroughputs matched to commercial FTIR spectrometers, i.e. in thevicinity of 1–10 mm²-stearadian. Commercially available IR fiber optics(multimode cylindrical waveguides made of, e.g., chalcogenide glass),have more recently been used as EWS sensors. These optical fiberstypically have much lower throughputs than the prism MREs, complicatingsomewhat their use with commercial IR spectrometers. Nevertheless, whenproperly coupled to a small-area (low-noise) IR detector, fiber opticsdisplay the advantage that miniaturization enables smaller amounts (μL)of sample to be detected. This advantage arises from the fact that,while the surface sensing area is smaller, the light experiences alarger number of reflection per unit length of waveguide, yielding aconcomitant increase in evanescent path length. It would be desirable tosee how far this advantage could be extended, i.e. how thin an EWSwaveguide or fiber could be made. However, it becomes impractical tomake a free-standing IR fiber less than ˜50 μm in diameter.

Most thin planar waveguide development has been in the visible region,where low-loss transparent materials (polymers and glasses) arecommercially available and easy to manipulate. Such waveguides havegenerally been used in conjunction with single-frequency lasers, whichprovide high luminosity, monochromaticity, and fine control over thelaunch angle, and have been used for absorption, Raman, and fluorescenceanalytical methods. In contrast, IR-transmissive materials with therequisite high refractive indices and low attenuation values are eithervery brittle, or have not had techniques developed to allow them to bedeposited (e.g. by evaporation or sputtering techniques) as uniform andwell-adhered films of the desired thicknesses of 1–100 μm.

BRIEF SUMMARY OF THE INVENTION

The disclosed supported planar and tapered, quasi-planar waveguidesprovide increased broadband transmission and demonstrate many of thecharacteristics predicted by planar waveguide theory. In particular,they show a great increase in the sampling sensitivity as compared toprevious evanescent-wave absorption measurements.

There are three particularly novel aspects to the fabrication and use ofthe disclosed thin supported planar IR waveguides. First, the waveguideshave been generated by physically “whittling away” at a macroscopicpiece of highly transparent single-crystal materials, such as Ge.,rather than by attempting either to deposit or to modify chemically athin film of transmissive material. The latter are the most commonapproaches for generating thin-film waveguides. For example, sputteringis the only method to have been used previously in an attempt tofabricate thin-film Ge light guides for wavelengths in the 2–10 μmrange. However, this attempt results in a waveguide with rather highattenuation of about 20 dB per cm, due to scattering from thenon-uniformly-deposited Ge. It is possible to detect transmission of CO₂laser light through such a waveguide, however attempts at detectingbroadband transmission through similarly-fabricated thin-film-sputteredwaveguides, e.g. 1-μm thick Ge on CaF₂, have failed. This is likely dueto the much lower luminosity of the broadband light source available, ascompared to the CO₂ laser used in prior art testings. The discloseddevices enable success in obtaining IR transmission using the weakerbroadband source through the development of waveguides with much lowerscattering losses than currently are obtainable with sputtered Ge films.

The disclosed waveguides further have an added “cladding” for thewaveguide's supported surface, in the form of a rather thickvacuum-deposited layer of a cladding material such as ZnS. This turnsout to be crucial for fabrication and use, since it is difficult toattach the piece of bulk single-crystal Ge to a substrate without usingIR-absorbing adhesive materials. Only by protecting the Ge with thevacuum-deposited cladding is it possible to use simple cements oroptical adhesives to attach it firmly enough to allow grinding andpolishing to a few-μm thickness.

The disclosed waveguide further uses a direct method to couple lightinto and out of its ends. Such direct coupling is generally not used formonochromatic (e.g. laser) light; more efficient coupling methods exist(e.g. prism coupling) that depend on optical interference effects.However, use of curved mirrors with foci at the two ends of a waveguideis probably the most generally useful means of coupling a broadbandwidth of light into it. This has long been known to be true formacroscopic MREs used for EWS. This is also be true for waveguides ofarbitrarily thin dimension, although in some thickness ranges, thewaveguides show considerable oscillations of throughput, as disclosedbelow.

Supported planar IR Ge waveguides, having a thickness between 50–100 μm,are useful as mid-IR evanescent-wave sensors. A significant portion ofthe light energy transmitted through such waveguides actually propagatesoutside the germanium, as an evanescent wave in the surrounding medium.With <100-μm-thick waveguides, a very small number of IR— absorbingmolecules near the surface of the waveguide can significantly attenuatethe light transmitted through the waveguide, allowing the measurement ofan ATR (attenuated total reflection) spectrum. Sizable ATR bands aretherefore observed from thin surface layers under 1 mm² in area. Thisincludes thin coatings on small pieces of polymer film, as well aspatches of the plasma membrane of large individual cells, e.g. frogoocytes.

One difficulty with using thin planar Ge waveguides as internalreflection elements (IREs) is coupling measurable amounts of lightthrough such waveguides and onto a detector. Prior art requires the useof an IR microscope in order to measure useful spectra throughwaveguides having a thickness between 30–100 μm. Use of a microscope,however, results in significant limitations on the waveguideconfigurations that can be used. In particular, waveguide lengths weregenerally limited to ˜12 mm, the maximum separation between objectiveand condenser focal points on commercial FTIR microscopes. Furthermore,the waveguides had to be positioned vertically, i.e. along the opticalaxis of the microscope. This is an inconvenience for samples containingliquids, especially small biological samples.

A quasi-planar waveguide, preferable made from single-crystal germanium,is also disclosed wherein one of the surfaces has an arcuate contourwhile a parallel, second surface is planar, the first surface beingconcave relative to the second surface. The perimeter is comprised ofmultiple opposing planar surfaces at right angles to the second surface.The second surface is coated with a cladding, such as ZnS and thenadhered to a substrate, such as quartz. The substrate must have aperimeter at least equal to that of the waveguide and a thicknesssufficient to support the waveguide. The arcuate surface of thewaveguide has an apex at least about four times greater than the nadir,with a preferred ratio of nadir to apex taper of at least about 1:10 andup to about 1:50. For clarity and consistency, the ratio can also bereviewed from the reverse standpoint, that is, apex to nadir. Thus, theratio of apex to nadir is up to 0.25:1. The preferred ratio of nadir toapex is in the range from about 0.01:1 to about 0.25:1. The nadir of thewaveguide is less than 100 μm, and preferably in the range of 1 to 20μm. The arcuate surface is polished to about a 0.1 μm finish to preventlight scattering. The tapered waveguide can also be coupled directly toan IR detector, eliminating the need for a microscope and enabling moreaccurate alignment. The elimination of the microscope also enables thewaveguide to be mounted horizontally. The tapered waveguide increases IRsignal throughput by about 4–5 fold, a result of filling the largenumerical aperture of a high-index waveguide medium (Ge, n=4). Thisincrease, for a given sensor thickness, makes it possible to detect theIR signal level more precisely in a shorter length of time. With anuntapered planar waveguide, the largest numerical aperture that can beattained inside the waveguide is equal to the numerical aperture of theelement that focuses light through air onto the end of the waveguide.This must always be less than 1, and for commercially available focusingoptics is typically 0.5–0.8. On the other hand, the fundamentallimitation on the largest numerical aperture that can be propagatedinside a dielectric waveguide is the refractive index of the waveguidematerial and it's cladding, and is equal to (n₁ ²–n₂ ²)^(1/2). Here n1is the refractive index of the waveguide medium (n₁=4 for Ge), while n₂is the highest refractive index of the cladding materials in contactwith the waveguide (n₂=2.26 for ZnS). For the disclosed ZnS-clad Gewaveguide, this maximum numerical aperture is 3.3, or approximately4-fold higher than the numerical aperture of available focusing optics.In theory, at least ˜4-fold more light energy can be propagated throughthe sensing region of a planar Ge waveguide than can be obtained byfocusing light through air into the edge of an untapered waveguide ofthe same minimum thickness. This theoretical maximum throughput is, infact, closely approached with the tapered waveguide design.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of the supported planar Ge waveguide used forinfrared evanescent-wave sensing;

FIG. 2 illustrates the separation Δf_(TM-TE) between the oscillatorytransmission patterns for TE and TM modes, normalized to the common modespacing shared by both, and plotted as a function of θ₁, the internalpropagation angle measured relative to the waveguide surface plane;

FIG. 3A graphs the uncorrected FT-IR single-beam intensity throughputspectrum for a typical 50-μm-thick waveguide with 15° bevel angles

FIG. 3B graphs the intensity spectrum, corresponding to FIG. 3A, of arectangular aperture set to the same size as the cross-section as thewaveguide (2 mm×50 μm);

FIG. 3C graphs the Ge spectral high and low frequency cutoffs;

FIG. 4A Fourier transforms of the single-beam intensity throughput ofthe 50-μm-thick waveguide with θ₂=15°, 30°, and 45° bevels;

FIG. 4B graphs the TE-polarized light of FIG. 4A;

FIG. 5A graphs the FT-IR evanescent-wave absorbance spectra of a 1-μLD₂O droplet on the waveguide for each of the three bevel angles (θ₂=15°,30°, and 45°);

FIG. 5B graphs the absorbance at 2650 cm⁻¹;

FIG. 6 is a schematic representation illustrating an IR light path fromthe source of the IR light, through the objective of an IR microscope,through a germanium waveguide, and showing the light collected by thecondenser mirror and focused onto a detector;

FIG. 7 is a schematic diagram of a tapered quasi-planar Ge waveguide andit's coupling to an IR detector;

FIG. 8 is schematic of an optical arrangement used to observe broadbandIR transmission or attenuation spectra through tapered quasi-planar Gewaveguide;

FIG. 9A is a graph illustrating the transmission properties of thedisclosed 20-μm-thick tapered waveguide;

FIG. 9B is a graph illustrating a transmittance noise spectrum using thedisclosed tapered waveguide;

FIG. 9C is a graph illustrating the cutoff of transmission at 5100cm-¹using a prior art planar waveguide;

FIG. 10A is a graph illustrating the attenuated total reflection (ATR)spectra of a liquid sample obtained with the disclosed tapered20-μm-thick waveguide;

FIG. 10B is a graph illustrated the attenuated total reflection (ATR)spectra of a solid film sample obtained with the disclosed tapered20-μm-thick waveguide;

FIG. 11 is an alternate embodiment of the tapered waveguide

FIG. 12 is a graph illustrating the comparison spectra between acetone,rubber cement and Scotch® Tape using the waveguide of FIG. 11;

FIG. 13 illustrates the absorbance sensitivity of the waveguide of FIG.11 for three thickness; and

FIG. 14 is a graph of halorhodopsin using the waveguide of FIG. 11.

DETAILED DESCRIPTION OF THE INVENTION

The waveguides manufactured herein are from germanium prisms, however asthe advantages over prior art waveguides are obtained through thescience rather than the materials, other elements can be substituted.For example, silicon or cadmium tellurium, will behave similarly,although the mechanical properties of these, and other, materials willrequire attention to procedures. For example, CdTe is significantly morebrittle than Ge and therefore requires additional care during thegrinding procedures. When selecting a cladding layer or substrate, thephysical properties in relation to one another and to the waveguidematerial must be taken into account. For example, when selecting acladding material, the strength of attachment to the waveguide and tothe cement/substrate must be considered. Selection of a substrate musttake into consideration the rigidity, optical transparency in the UV andthe ability to reach a high degree of flatness in surface polish. TheGe/ZnS/quartz combination disclosed herein provides an example of thedesirable material interaction and can be used as a baseline forcomparison.

Additional disclosure is contained in applicants' publication entitled,“Design for Supported Planar Waveguides for Obtaining Mid-IREvanescent-Wave Absorption Spectra from Biomembranes of IndividualCells”, Mark S. Braiman, and Sysan E. Plunkett, Volume 51, Number 4,April 1997, Applied Spectroscopy, copyright 1997, Society for AppliedSpectroscopy, the disclosure of which is incorporated herein, byreference thereto, as though recited in full.

Another additional disclosure is contained in applicants' publication,entitled, “Mid-IR evanescent-wave absorption spectra of thin films andcoatings measured with a ˜50-micrometer planar Ge waveguide sensors”,James J. Stone, Mark S. Braiman, and Susan E. Plunkett, published Jun.15, 1997, Process SPIE, the disclosure of which is incorporated herein,by reference thereto, as though recited in full.

Fabrication of Thin Supported Planar Waveguides.

Infrared waveguides were fabricated from commercially available prismsof Ge and ZnS. The Ge prisms were purchased as 12×2×2-mm orthorhombsfrom Spectral Systems, and were each coated on one 12×2-mm side with a2-μm-thick layer of ZnS using chemical vapor deposition (CVD). TheZnS-coated side of each Ge prism was then cemented withpolycyanoacrylate adhesive to a ZnS substrate (25×12×2-mm orthorhomb).As shown by repeated failed attempts to transmit light through uncoatedwaveguides, the IR-transparent layer between the Ge waveguide and theadhesive is absolutely necessary to prevent the IR light from beingcompletely attenuated by the strongly absorbing polycyanoacrylate.

The 2 mm-thick supported Ge strip was then ground and polished by handto a final thickness of 30–100 μm using the abrasive powders and flatglass polishing stone in a commercially available polishing kit(Harrick, Ossining, N.Y.). For the final steps of polishing, theabrasive powders were replaced by Al₂O₃ lapping paper. The finalthickness and degree of polish of each waveguide were measured with avisible light microscope. Typically the observed random surfacescratches were less than 3 μm in depth. Beveled ends were ground on thewaveguide (and the substrate below) using PTFE guides cut to the desiredangle, using the same lapping paper. A schematic of a typical waveguideis shown in FIG. 1 where θ₁ is the internal propagation angle; θ₂ is thelaunch or bevel angle; and n₁, n₂, n₃ are the refractive indices of thewaveguide, substrate, and superstrate, respectively.

Broadband infrared light was focused through the waveguide and measuredusing an IR microscope (IR-Plan™ Infrared Microscope Accessory,Spectra-Tech, Stamford, Conn.), interfaced to an FT-IR spectrometer.This IR microscope was selected because it is one of the only modelsavailable that permits the separate focusing of the objective andcondenser mirrors on the input and output ends of the waveguide, some 12mm apart. Light exiting the waveguide was collected and focused onto aphotoconductive HgDcTe dectector having (0.1 mm)² active area (GrasebyInfrared, Model FTIR-M 16-0.10). Data processing was done with GRAMS 386software (Galactic Industries, Salem, N.H.).

Oscillations in the single-beam throughput spectrum of a planarwaveguide arise from the requirement to satisfy one of the twoeigenvalue equation one of the two eigenvalue equations of a planarwaveguide in order to obtain transmission. For a defined thickness andpropagation angle, each of these two equations (see below), whichcorrespond to the two possible polarizations, is satisfied only at a setof evenly-spaced light frequencies. The spacing is the same for bothpolarizations, and is thus observed even when observing the transmissionwith unpolarized light.

This was achieved by starting from a standard theory of planardielectric waveguides, and deriving expressions that relate the periodof the oscillations (Δ v) in the broadband IR transmission spectrum tothree experimentally fixed parameters of the waveguide: thickness d,refractive index (n₁), and propagation angle θ₁ or bevel angle θ₂(defined by the diagram in FIG. 1). The planar waveguide sensors areconsidered as approximations to the well-studied asymmetric planar slabwaveguide, where waveguide, substrate, and superstrate have refractiveindices n₁, n₂, n₃, respectively. Using the shorthand notationsn₂₁=n₂/n₁ and n₃₁=n₃/n₁, then the eigenvalue equations are:

$\begin{matrix}{{\tan\;\kappa\; d} = {\frac{\kappa( {\gamma + \delta} )}{\kappa^{2} - {\gamma\delta}}( {{for}\mspace{14mu}{guided}\mspace{14mu}{TE}\mspace{14mu}{modes}} )}} \\{= {\frac{\kappa( {{n_{21}^{2}\gamma} + {n_{31}^{2}\delta}} )}{{n_{21}^{2}n_{31}^{2}\kappa^{2}} - {\gamma\delta}}( {{for}\mspace{14mu}{guided}\mspace{14mu}{TM}\mspace{14mu}{modes}} )}}\end{matrix}$

The parameters κ, γ, and δ are characteristic of the mathematicalsolutions to Maxwell's equations in the waveguide, the substrate, andthe superstrate, respectively. For waveguides that are thick compared tothe wavelength of light propagating inside them, these variables can beapproximated as simple functions of a well-defined propagation angle θ₁.To enable the defined propagation angle, only a spectral region ofsufficiently short wavelength (<10 μm in vacuo, or <2.5 μm inside theGe), compared to the waveguide thickness d (30–50 μm) was considered. Wewill therefore make the substitutions κ=2πn₁ vsin θ₁; γ=2π v(n₁²cos²θ₁−n₂ ²)^(1/2); δ=2π v(n₁ ²cos²θ₁−n₃ ²)^(1/2). These are equations1.3-26, 1.3-63, and 1.2-13 through 1.2-15, 1, with minor mathematicalrearrangements, as found in Theory of Dielectic Optical Waveguides,Marcuse, D., Academic Press, N.Y., 1991.

Hereinafter it is disclosed that, because of the very high refractiveindex of Ge (n₁=4.0), the propagation angle θ₁ is almost equal to thebevel angle θ₂ of the ends of the waveguide, regardless of what range ofangles of light are focused on the input end and collected from theoutput end of the waveguide. Therefore a value of θ₁ is used, calculatedby assuming simple Snell's-law behavior for a central (axis) ray of themicroscope's light-focusing mirror system, i.e. θ₁=θ₂−arcsin[sin(θ₂/n₁)]≅0.75θ₂.

The eigenvalue equations can now be re-cast in terms of the experimentalparameters v=1/λ (the wavenumber of the light); the propagation angleθ₁; and the refractive indices n₁, n₂, and n₃. From the resultingsimplified eigenvalue equation, the allowed solutions of v at externallyfixed values of θ₁ and d can be obtained. Note that this differs fromthe more common approach of examining the solutions of θ₁ at fixedvalues of v and d.

The right sides of equations 1 and 2 are both independent of lightfrequency v, since every factor of this parameter in the numerator isbalanced in the denominator. Thus, the two eigenvalue equations reducetotan κdε{E_(TE), E_(TM)}where

$E_{TE} = {\frac{\sin\;{\theta_{1}\lbrack {( {{\cos^{2}\theta_{1}} - n_{21}^{2}} )^{1/2} + ( {{\cos^{2}\theta_{1}} - n_{31}^{2}} )^{1/2}} \rbrack}}{{\sin^{2}\theta_{1}} - {( {{\cos^{2}\theta_{1}} - n_{21}^{2}} )^{1/2}( {{\cos^{2}\theta_{1}} - n_{31}^{2}} )^{1/2}}}\mspace{20mu}{and}}$$E_{TM} = \frac{\sin\;{\theta_{1}\lbrack {{n_{21}^{2}( {{\cos^{2}\theta_{1}} - n_{21}^{2}} )}^{1/2} + {n_{31}^{2}( {{\cos^{2}\theta_{1}} - n_{31}^{2}} )}^{1/2}} \rbrack}}{{n_{21}^{2}n_{31}^{2}\sin^{2}\theta_{1}} - {( {{\cos^{2}\theta_{1}} - n_{21}^{2}} )^{1/2}( {{\cos^{2}\theta_{1}} - n_{31}^{2}} )^{1/2}}}$are simply two constants determined by the parameters θ₁, n₁, n₂, and n₃for a particular waveguide geometry. The set of solutions to thesimplified form of the eigenvalue equation is now easily obtained:

κ d = arctan   E + π N; E ∈ {E_(TE), E_(TM)}${\overset{\_}{v} = \frac{f + N}{2n_{1}d\;\sin\;\theta_{1}}};{f \in \{ {\frac{\arctan\mspace{14mu} E_{TE}}{\pi},\frac{\arctan\mspace{14mu} E_{TM}}{\pi}} \}}$

In both of the preceding equations, N is allowed to take on any integervalue. That is, the allowed TE and TM frequencies are each expected tobe evenly spaced, with a period of Δ v=1/(2n₁d sin θ₁). The calculatedseparation between the TE and TM series, Δf_(TM-TE)=(arc tan E_(TE)−arctan E_(TM))/π, is expected to be 0 for θ₁=0, and to increase roughlylinearly with θ₁, until very close to the critical angle. For thematerials used by us (n₁=4.0, n₂=2.2, n₃=1), Δf_(TM-TE) is plotted as afunction of θ₁ (in radians) in FIG. 3.

Depending on the separation between TE and TM modes, it is easier ormore difficult to see their shared oscillation period in the throughputspectrum obtained with unpolarized light. At low values of θ₁, where theTE and TM modes are expected to be separated by much less than a singleoscillation period, they should superimpose quite well, making it easyto see an interference pattern. At values approaching the criticalangle, however, the TE and TM modes are expected to be almost perfectlyinterleaved, leading to an apparent period that is only half of theactual period 1/(2n₁d sin θ₁) and to a smaller-amplitude intensityoscillation that is much harder to observe on the gradually-changingthroughput spectrum. This actually turns out to be quite desirable for abroadband evanescent-wave sensor.

FIG. 3A shows the uncorrected FT-IR single-beam intensity throughputspectrum for a typical 50-μm-thick waveguide with 15° bevel angles. Itis compared with the open-beam throughput spectrum of the microscopethrough a rectangular aperture the same size as the cross-section of thewaveguide (2 mm×50 μm) shown in FIG. 3B. The sharply-delineated spectralfeatures present in both waveguide and open-beam spectra near 1650,2200, and 3800⁻¹ are absorption bands due to gaseous water and carbondioxide. These are present since the beam path in the IR microscopecontained room air, (i.e., was unpurged). FIG. 3C illustrates anexpansion of the 6000–4000 cm⁻¹ region, clearly showing the highfrequency transmission cutoff of Ge at ˜5400 cm⁻¹. The most obviousnovel feature in the waveguide throughput spectrum is therapidly-oscillating beat pattern, superimposed on the normal throughput,in the 2000–3500 cm⁻¹ region. As discussed further below, thisinterference pattern corresponds closely to the mode structure predictedby waveguide theory, and is the clearest demonstration that light isbeing guided though the thin layer of Ge. Additionally, the waveguideshows characteristic Ge spectral high and low frequency cut-offs at 5400cm⁻¹ (see inset) and 550 cm⁻¹.

It should also be noted that even below 5400 cm⁻¹, the spectralintensity transmitted through the waveguide decreases with increasingfrequency much faster (relative to the maximum value near 2000 cm⁻¹)than in the open-beam spectrum. This drop-off is an indication of thescattering losses due to imperfections on the waveguide surface(s). Theless thoroughly the surface of the waveguide was polished, the moredrastic was the drop-off. It would almost certainly be possible toimprove on the high frequency throughput, since commercial polishersroutinely obtain better finishes on optics than obtained by handpolishing. The overall measured transmittance of the waveguide at 2000cm⁻¹ is about 5% relative to an aperture of the same cross-section. Thereflection losses from the two air-Ge interfaces at the ends of thewaveguide are about ˜50%, based on measurements of the transmittancethrough a Ge window. Thus the waveguide has an attenuation of about 10dB over its entire 12-mm length. This means that the disclosed 50-μmthick Ge waveguide has about 10-fold less attenuation than a 1-cm-long,5-μm thick Ge waveguide sputtered onto a KRS-5 substrate, for which theattenuation was estimated as 20 dB per cm, and through which lighttransmission was detected only by using a powerful CO₂ laser.

FIG. 4A shows Fourier transforms of the 4400–2430 cm⁻¹ region of thethroughput spectra for bevel angles (θ₂) of 15°, 30°, and 45°. In each,the spike feature associated with the oscillatory (beat) pattern in thespectrum is indicated with an arrow. Each spectrum, measured as in FIG.4A, was truncated at 4400 and 2430 cm⁻¹, then apodized using aBlackman-Harris 3-term function, and Fourier transformed. The phase wascorrected to obtain just the amplitude of the Fourier transform. The 15°and 30° data were obtained with unpolarized light. However, as θ₁increases, the amplitude of the oscillatory pattern in the spectrumdecreases, because the TE- and TM-mode beat patterns move “out of phase”and cancel each others' intensity. Therefore, data at 45° were obtainedusing TE-polarized light (using a wire grid polarizer). With unpolarizedlight at 45°, the spike in the corresponding plot is just barelyvisible, at nearly the same point as obtained with the TE-polarizedlight. FIG. 4B plots the reciprocal of the oscillation period (1/Δ v)versus internal propagation angle (θ₁). The filled circles areexperimental data and the straight line is the theoretically predictedbehavior: 1/Δ v=(2n₁d sin θ₁) with n₁=4.0, d=50 μm, and θ₁=θ₂−arcsin[sin(θ₂/n₁)]. The main source of error in this plot was imprecisionin grinding the bevel angle θ₂ The error bars in the inset show theresulting ±5° uncertainty in θ₁.

These plots provide the most precise measurement of the period of theoscillating beat pattern, since a sine wave in the spectrum correspondsto a spike in its Fourier transform. The optical retardation at thisspike is just the reciprocal of the oscillation period Δ v in thespectrum. The inset is a plot of the reciprocal of the oscillationperiod (1/Δ v) versus internal propagation angle (θ₁). The filledcircles are experimental data and the straight line is the theoreticallypredicted behavior using Equation (3) above for unpolarized light: 1/Δv=(2n₁d sinθ₁) with n₁=4.0, d=50 μm, and θ₁=θ₂−arc sin[sin(θ₂/n₁)]. Itis apparent that there is a close correlation between experimental andtheoretical values.

FIG. 5A shows absorbance spectra for a ˜2 mm-diameter D₂O droplet on thewaveguide for each of the three bevel angles. D₂O (deuterated water) waschosen since it adheres well to the waveguide, evaporates slowly, andexhibits well-known absorption bands in spectral regions unobscured byabsorption due to H₂O vapor. Bands at ˜2500 cm⁻¹ and 1250 cm⁻¹ are dueto D-O stretch and DOD bend vibrations, respectively. The smaller bandsat 3400 cm⁻¹ and 1450 cm⁻¹ are due to H-O stretch and H-O-D bendvibrations, and resulted from rapid H/D exchange of the droplet with H₂Oin the room air over the course of the 30-minute measurement. The degreeof exchange was similar for all 3 measurements, as was the decrease indroplet size (20% –30% over 30 min) due to evaporation. FIG. 5B plotsthe absorbance at 2650 cm⁻¹ versus internal propagation angle θ₁. Thefilled circles are experimental data and the straight line is thetheoretically predicted behavior for TE-modes (see text). The A₂₆₅₀values were each increased to take into account the absorbance at ˜3500cm⁻¹ resulting from H/D exchange. The horizontal error bars representour estimate of ±5° uncertainty in the bevel angle; the vertical errorbars result from noise in the spectrum and uncertainty in the degree ofH/D exchange. As the bevel angle increases, surface sensitivity(detected IR absorbance per unit sample contact area) also increases.This phenomenon is the result of three well-established relationships ofthe bevel angle (θ₁) to detected intensity: (1) the evanescent fieldpenetration depth (d_(p)) increases with θ₁ up to θ_(critical); (2) theinterfacial evanescent field intensity increases monotonically with θ₁,up to 90°; and (3) the number of internal reflection increasesmonotonically with θ₁. At low angles θ₁, the measured absorbance isexpected to be roughly a quadratic function of sin θ₁. The measured IRabsorbance A is related to known parameters of the water (D₂O) sampleand waveguide jby multiplying the right side of Harrick's equation 2–25,which describes the coupling of the evanescent wave to an absorbingmedium at a single internal reflection, by the number of internalreflections at which the D₂O droplet is sensed. This number is tanθ₁×l/2d (remembering that the absorbing medium is present on only oneside of the waveguide). For simplicity, we assume the use ofTE-polarized light. Corresponding expressions for TM-polarized orunpolarized light are somewhat more complicated but of a similarmagnitude, and exhibit a roughly similar dependence on θ₁.

$A = {\frac{k_{3}n_{31}^{2}l}{( {1 - n_{31}^{2}} )d}\frac{\sin^{2}\theta_{1}}{\cos\;{\theta_{1}( {{\cos^{2}\theta_{1}} - n_{31}^{2}} )}^{1/2}}}$

Here k₃ is the imaginary refractive index of the sample (estimated forD₂O at ˜150 cm⁻¹ above its 2500-cm⁻¹ absorption maximum, by using apublished value of 0.13 for H₂O at a corresponding frequencydisplacement from its 3350-cm⁻¹ absorbance maximum); n₃₁ is the ratio ofthe (real) refractive index of the sample to that of the waveguide,0.33; l is the contact length of the D₂O droplet with the waveguidesurface (2.5 mm); d is the waveguide thickness (50 μm); and θ₁ is theinternal angle of propagation, which we varied.

The inset to FIG. 5 is a plot of IR absorbance at 2650 cm⁻¹ (A₂₆₅₀)versus internal propagation angle (θ₁). The filled circles areexperimental data and the straight line is the theoretically predictedbehavior. For this plot, a wavenumber somewhat away from the absorbancemaximum was selected, to reduce problems due to absorbance flattening.This is a well-known phenomenon in EWS that arises due to the inaccurateassumption of only a single internal propagation angle θ₁, and only asingle contact length l for the roughly-circular water droplet. In fact,the use of focusing optics with large numerical aperture means that foreach bevel angle θ₂, light traversing the waveguide has a range ofinternal propagation angles θ₁. Furthermore, the interaction length l issignificantly shorter for light traversing the waveguide near the edgesof the 3-mm-diameter droplet than for light near the center. Both ofthese factors mean that there is actually a range of effective pathlengths through the sample in each of the measurements. This results ina sublinear dependence of absorbance on average effective path length,i.e. a non-Beer's Law type of behavior, as we actually observe. Thedeviation of measured data from theoretical dependence on θ was evengreater when a wavenumber closer to the absorbance maximum of 2500 cm⁻¹was selected (not shown).

The large surface sensitivity demonstrated in FIG. 5 is a significantimprovement over previous studies using optical fibers forevanescent-wave IR spectroscopy. For instance, Simhony et al, achievedan absorbance of only 0.5 for the most intense band in the H₂O spectrum(3350 cm⁻¹); using an immersion length of 65.5-mm for a 900-μm diametersilver halide (AgCl_(x)Br_(1-x)) fiber optic in water. The sameabsorbance value (0.5) was obtained for a 70-mm length of 500-μmdiameter chalcogenide fiber, using a different coupling method thatresulted in a different set of propagation angles θ₁ than in the silverhalide fiber experiment cited. The vast increase in sensitivity in thecurrent study is due to the thinness (d) of the waveguide, as well asthe ability to polish its supported ends at a bevel angle (θ₂) of up to45°. As mentioned above, the number of internal reflections per unitlength varies as tan θ₂/d. Therefore, a 10-fold reduction in thickness(500 μm to 50 μm), and an increase of θ₂ from 10–15° maximum for afree-standing fiber to 45–50° for our supported waveguide, has yieldedover a 30-fold decrease in the sample contact length required to obtainan absorbance reading of 0.5.

Tapered Quasi-Planar Waveguides

The thin, mid-IR-transmitting, waveguide sensors disclosed overcome theprior art difficulties in light coupling light through use of a gradualbi-directional taper. Tapering has been used for some years as a meansof improving the optical throughput of small cylindrical waveguidesensors, e.g. glass optical fibers. Cylindrical fiber tapered waveguides can be produced by melting/softening and drawing, an approachthat is not directly applicable to planar Ge waveguides. To produce atapered thin planar waveguide is technically more difficult thantapering a cylindrical chalcogenide fiber, especially when the goal isto achieve a sensor thickness below 100 μm.

The melting/softening and drawing combination has been used for years toproduce tapered shapes for waveguides as well as glass micropipettes,etc. The drawing process, when applied to a softened region of a pieceof glass of arbitrary shape, tends to produce a taper that is more andmore cylindrically symmetrical the longer the drawing is carried out.There is no comparably simple process for generating a quasi-planarwaveguide shape from a softened piece of glassy material.

The simplest procedure would be to roll a softened piece of glassymaterial against a hard surface. A tapered thickness is produced by thisprocess, but with nowhere near the surface polish that is attainable fora drawn glass taper of cylindrical symmetry. An additional problem isthat the resulting “waveguide” has irregular edges, which cause problemsin the throughput. Ideally, a flat nearly planar waveguide should havelinear, or perhaps smoothly curved, edges. Thus, it would appear thatcylindrical fiber tapered waveguide concepts are not applicable tonon-cylindrical, non-fiber waveguides.

The disclosed tapered, “quasi-planar,” waveguides have properties thatmake them particularly useful for certain types of mid-IRevanescent-wave sensors. The term “quasi-planar” as employed herein,refers to a waveguide that has a single planar surface, and a secondary“quasi-planar” parallel surface. The quasi-planar surface deviates froma true planar surface in that it is an arcuate. This tapering improvesthe efficiency of the optical coupling both into the waveguide from anFTIR spectrometer, and out of the waveguide onto a small-area IRdetector. The tapering further enables the elimination of an IRmicroscope to couple light through the waveguide. Instead, it ispossible to obtain extremely efficient coupling with a detector directlycoupled to an immersion lens. This optical arrangement enables thedisclosed tapered waveguides to be useful as sensors, because itsimplifies the positioning of optical accessories needed to couple lightinto the waveguide. Untapered waveguides require a microscope or otherbulky focusing mirrors close to the waveguide, thereby blocking easyaccess to its surface for depositing materials to be analyzed.Additionally, the elimination of the IR microscope permits the sensorsto be mounted horizontally, an added advantage when using liquids.Furthermore, using a Ge waveguide having a 20- μm×1-mm cross section,sufficient throughput is obtained to give signal/noise ratios in excessof 1,000 over most of the 1000–5000 cm-¹ range, with two (2) minutes ofscanning at 8 cm-¹ resolution. The small (0.02 mm²) cross section of thewaveguide yields great sensitivity to small numbers of IR-absorbingmolecules near its surface. The optimum thickness for the waveguide is1-μm, however due to the output obtained with the 20-μm waveguides, inmany applications the increased output obtainable by 1-μm will notprovide any advantages.

As illustrated in the schematic diagram 170 of FIG. 7, the disclosedquasi-planar Ge waveguide 172 has been coupled to an IR detector 176,such as sold by Remspec Instruments, model MOD-02. The focused inputlight 174, shown at left of the figure, is typically from an FTIRspectrometer. In this embodiment, one of the flat surfaces of thewaveguide 172 is first coated with a thin cladding layer of ZnS, or anequivalent coating, then cemented to a rigid substrate 178, such asquartz. The top, unadhered, surface of the waveguide 172 is ground to alarge-radius arcuate shape having a cylindrical sector of radius ˜300mm. Preferably a commercial tool for grinding concave cylindrical lensesis used to grind and polish the waveguide, to enable the accuratetapering of the prism.

Although the end thickness, or apex, is not necessarily critical, theapex should be at least 4-fold thicker than the minimum thickness, ornadir, at the middle. The ends should also have a thickness no greaterthan the width of the waveguide for optimum optical performance usingcommercially available IR detector elements, which are square orcircular. To obtain optimum performance, the waveguide end should beimaged onto the detector without any overhangs.

Fabrication Of Tapered Quasi-Planar Waveguides

Tapered Ge waveguides are fabricated using modifications of previouslypublished procedures, using a commercial tool for grinding the concavecylindrical lenses. Greater care is needed to avoid scratching thewaveguide surface as it is more difficult to fix any scratch or gougeonce it has occurred. This greater care includes in the selection andmaintenance of grinding/polishing surfaces.

To make the tapered waveguides shown in FIG. 7, custom polished 50×20×1mm Ge prisms are used as the starting material. They are coated on oneface with a 1.2-μm-thick CVD coating of ZnS and then cemented to50×50×2-mm quartz substrates using a UV-curing optical adhesive. Thesubstrate dimensions must be at least that of the prism to providesupport, however, the dimensions beyond the periphery of the prism aredetermined by end use and convenience in handling. To form the taperedsurface, the waveguides are ground using aluminum oxide grinding powdersagainst a commercially available cylindrical grinding tool with anappropriate diameter. The coarser techniques are used until the taperedportion has almost reached the desired thickness. Pads, designed for usewith curved surfaces, are used with the grinding tool and the powders tocreate the grinding/polishing surface. The thickness of the middle canbe determined by observing the interference pattern between reflectionsfrom the front and back surfaces of the Ge waveguide in an FTIR spectrumwith an IR microscope in reflectance mode. At that point, the curvedsurface is polished with a slurry combination of aluminum oxide (12.5μm) and diamond powder (0.1 μm) and particle embedded soft films.Careful fine polishing to a 0.1- μm, or below, finish is crucial forminimizing light scattering from imperfections in the surface. Toaccomplish the required finish, the films are covered with water duringthe polishing process with the particle size within the embedded filmsdecreasing with each polishing, i.e. 12.5, 9, 6, 3, 1, 0.5, 0.3 and0.1-μm.

In the event the Ge prisms are not available at the desired endthickness, the prisms can be ground against a flat glass, or equivalentgrinding stone, to the final thickness. The curved surface is thenground and polished as set forth above.

In FIG. 8 collimated light output by a commercial FTIR spectrometer 182with a blackbody source is focused along a horizontal optical axis, intothe 1-mm² entrance aperture 190 of the vertically placed waveguide 186,by using a single off-axis paraboloid mirror 184 (f=25 mm). Alignment atthe output end 192 of the waveguide 186 is greatly simplified by using aRemspec immersion detector 188, which has a short focal lengthIR-transmitting lens 194 directly in contact with the small-area HgCdTedetector 196. The output end 192 of the tapered waveguide 186 is placedas close as possible to the lens 194 and along its axis.

As illustrated in the graphs of the optical arrangement shown in FIGS. 9and 10, broadband optical throughputs sufficient to saturate the Remspecdetector/preamp combination 188 are easily achieved through a planar Gewaveguide of 20-μm thickness. Spectral measurements through thiswaveguide, measured with 8-cm-1 resolution over a bandwidth of 0–7900cm-¹, have a signal/noise ratio in excess of 1000 after only 2 min scantime (see FIG. 9). This signal/noise ratio applies over the range1000–2500 cm-¹.

However, there are some notable differences in the signal/noise ratio atsub-regions within the spectral range of 1000–2500 cm-¹. The relativelylarger depletion of light at higher frequencies is due to greaterscattering losses from imperfections at the surfaces of the thinnerwaveguide. Since all surface phenomena are magnified with a thinnerwaveguide, it is crucial to polish the tapered waveguide surface asthoroughly as possible.

The light spectrum transmitted through the 20-μm thick taperedwaveguide, and graphed in FIG. 9, is similar in most respects to thattransmitted through flat Ge planar waveguides. The disclosed waveguide,however, increases the total amount of light transmitted through thewaveguide, per unit cross-sectional area at the waveguide's thinnestpoint, by 4–5 fold greater than a planar waveguide.

The intensity spectrum from FTIR spectrometer with broadband blackbodyIR source, shown in 9A, uses an HgCdTe detector, and reflects datagathered from the arrangement of FIGS. 7 and 8. In particular, thedistinct cutoff of transmission near 5100 cm-¹, as illustrated in by theinset of FIG. 9C, is characteristic of light transmitted through aprior-art planar Ge waveguide. The sharp attenuation bands near 2300 and2000–1400 cm-¹ are due to atmospheric CO₂ and H₂O vapor, respectively,in the open path of the IR beam. In 9B, 100% transmittance noisespectrum calculated from the ratio of two successive single-beamintensity spectra, each acquired in 2 min (1000 scans) with 8-cm-¹resolution is graphed. The tapered waveguide has a broad intrinsicabsorption band near 3500 cm-¹ that leads to substantial baselineirregularities in ratioed spectra (FIG. 9B). The peak-to-peaktransmittance noise in the 2000–2200 cm-¹ range is 0.1%. Thepeak-to-peak transmittance noise from the tapered waveguide is less than0.2 as large as that measured in the same amount of time using a planarwaveguide with a similar thickness.

The transmission spectrum of the tapered waveguide is especially uniquefor a feature that it lacks, namely the oscillatory interference patterncharacteristic of planar waveguides with a fixed thickness andpropagation angle. The observed oscillations in transmitted intensityarise from a fixed frequency separation between allowed waveguide modes.With the tapered waveguide, there is a wide range of propagation anglesas well as a wide range of waveguide thicknesses. This results in asuperposition of oscillation patterns continuously covering a wide rangeof different periods, i.e. no discernible oscillatory pattern at all.

FIG. 10A shows the attenuated total reflection (ATR) spectra of a liquidwhile FIG. 10B shows the ATR of a solid film sample. Both were obtainedwith a tapered 20-μm-thick waveguide and the light source and detectorillustrated in FIGS. 7 and 8. The two samples shown are deuterated water(D₂O) and a thin film of halobacterial membrane containing lipid (25%)and protein (75%). In each case, the spectrum presented is−log(I/I_(o)), where I is the intensity spectrum measured in thepresence of sample, and Io is the spectrum measured in its absence. Ineach case the sample covered a region ˜1 mm in area at the thinnestcentral region of the waveguide, the measurement time was 2 min (500scans), and spectral resolution was 8 cm-¹.

The D₂O sample was measured with a 1-μL droplet covering only a ˜1-mmlength of the thinnest portion of the waveguide. Coverage of longerregions of the waveguide produced only small increases in the size ofthe absorbance bands. (Data not shown). The graph of FIG. 10A shows astrong O-D stretch vibration near 2500 cm-¹ and weaker D-O-D bendingvibration near 1250 cm-¹.

The dried film of ˜2 pmol bacteriorhodopsin (60 ng purple membrane)sample was prepared by drying a 1-μL droplet of a suspension of purplemembrane fragments (50 μg/mL) onto the thinnest portion of thewaveguide. The 3 strongest bands near 1650, 1550, and 1200 cm-¹ are dueto amide I, amide II, and amide III vibrations, respectively, and arecharacteristic of the peptide backbone.

In comparison to prior art spectra of similar samples obtained using anIR microscope with planar waveguides 30–50 μm in thickness, the spectraof FIG. 10 have substantially improved signal-noise ratios for 10–20μshorter measurement times. For example, the noise level in both of thespectra of FIG. 10 (obtained with 500 scans each) is 0.001 absorbanceunits, whereas in spectra obtained with the microscope-coupled planarwaveguides, the noise level was typically 0.01 absorbance units for10,000 or 20,000 scans.

At the same time, the absorbance signals are somewhat reduced (between3- and 5-fold) for similar sized samples on the tapered 20-μm waveguide,as opposed to the untapered 30-μm waveguides with 45° bevels usedpreviously in the prior art. The reduction in attenuation signals is dueto the predominance in the tapered waveguide of light propagating atrelatively low off-axis angles, i.e. angles that lie less than 45° awayfrom the waveguide surface plane. Light in such modes is absorbedrelatively inefficiently by molecules at the surface, giving rise tosmaller attenuation signals per molecule.

The principal advantage of tapering thin Ge planar waveguides is topermit a substantial increase in throughput for a given sensorthickness, making it possible to detect the IR signal level moreprecisely in a shorter length of time. The increase in throughputresults from filling the large numerical aperture of a high-indexwaveguide medium (Ge, n=4). With an untapered planar waveguide, thelargest numerical aperture that can be attained inside the waveguide isequal to the numerical aperture of the element that focuses lightthrough air onto the end of the waveguide.

On the other hand, the largest numerical aperture that can be propagatedthrough a tapered waveguide is determined by the refractive index of thewaveguide material and it's cladding, and is equal to (n₁ ²–n₂ ²)^(1/2)Here n₁ is the refractive index of the waveguide medium (n₁=4 for Ge),while n² is the highest refractive index of the cladding materials incontact with the waveguide (n₂=2.26 for ZnS). For the disclosed ZnS-cladGe waveguide, this maximum numerical aperture is 3.3. Thus, ˜4-fold morelight energy can be propagated through the sensing region of a planar Gewaveguide than can be obtained by focusing light through air into the(untapered) waveguide edge.

Gradually tapering waveguide enables an increase in the numericalaperture. In such a taper, the product of the numerical aperture andwaveguide thickness remains constant, as long as the maximum numericalaperture of the waveguide is not exceeded. A cone of light withnumerical aperture of 0.3 that is transmitted into a 1-mm thick Gewaveguide maintains that numerical aperture across the air/Ge interface.Inside the Ge, it has a half-angular spread of only arcsine (0.3/4)=4°.It is possible to achieve a numerical aperture of 3.3 by graduallytapering the waveguide by a factor of about 10. That is, once awaveguide thickness of about 100 μm is reached, the numerical apertureof the waveguide is filled. At this thickness, the cone of propagatinglight rays extends all the way to the critical angle between Ge and ZnS,that is, to a half-angular spread of arcos (2.26/4)=56°.

The taper factor used herein (1 mm/20 μm=50) is much larger than theratio of the maximum numerical aperture of the waveguide (3.3) to thenumerical aperture of the input focusing optic (˜0.3). This excess taperfactor is intended to guarantee that, to the greatest extent possible,the numerical aperture of the sensing region of the waveguide is filled.With the particular light source present in the Midac spectrometer usedherein, it is not difficult to fill the 1×1 mm input aperture of thetapered waveguide. Thus, a significant fraction of the input light isexpected to be coupled out of the waveguide, i.e. to exceed the criticalangle, as the waveguide is tapered down to its minimum thickness. Itshould be noted that the optimum apex to nadir ratio is dependent uponthe detector size and shape and, when taken in conjunction with theteachings herein, will be apparent to those skilled in the art.

Much of the light that goes into one end of the waveguide is lost as ittravels into the middle (thinnest) portion of the waveguide, but then asthe light travels into the region where the waveguide tapers outwardagain, there is no further loss of light energy (or flux). The loss oflight is therefore not due simply to the presence of non-parallelsurfaces; but more specifically to the presence of surfaces thatconverge to a thickness less than ¼ of the input thickness. That is,nearly all the light present in the tapered region reaches the outputface of the waveguide.

From here, the light is efficiently focused onto the 100-μm×100 μm areaof the HgCdTe element in the Remspec detector. The use of an immersionlens in this detector provides an efficient coupling method that isextremely insensitive to the position of the fiber (or waveguide) outputend. This greatly simplifies waveguide alignment, relative to theprocedures that were required previously with a microscope. Whenincorporating a microscope, the output end of the waveguide had to bepositioned at the very small focal area of the microscope's objectivesince the IR signal could be lost entirely with a mispositioning of aslittle as 50 μm.

The 1-mm width of the waveguide used in the example herein was chosen asthe minimum width that could be easily manipulated without breaking. Thethickness at the ends in these examples is the same as the width tomatch the square shape of the IR detector element used in the testing.The prism was then tapered as stated heretofore. Various taper ratioswere tested with the result that the greater the thickness, the lowerthe sensitivity. A minimum 0.1-mm thickness, which corresponds to ataper ratio of 10, gave a high light throughput, but a lower (at least5-fold) sensitivity to analyte at the surface than the 20 μm thickness.Test data (not shown) showed a continuous increase in sensitivity as thethickness of the waveguide decreased.

The wide range of propagation angles present at the sensing area of thetapered quasi-planar waveguide eliminates the distracting oscillatorytransmission pattern that is observed for thin planar Ge waveguides.This is advantageous for a sensor, because it means that there are nosharp features in the spectrum that could be mistaken for absorptionbands of a material present at the waveguide surface. Furthermore, thetransmitted intensity at any frequency is not nearly as sensitive towaveguide alignment as with true planar waveguides.

The wide range of propagation angles present can lead to some degree ofnon-linearity of the absorbance signal, presenting small deviations fromlogarithmic response (i.e. the absorbance nonlinearities). Inparticular, the nonlinear response is not important for measurement ofdifferent spectra of samples that are subjected to an in situperturbation while they are adsorbed or adhered to the surface of thewaveguide. Additionally, the nonlinear response can be unimportant ifthere is a single known analyte, and a calibration curve can beestablished.

With the tapered waveguide, most of the internal reflections occurwithin a fairly small region near the point of minimum thickness. Thus,molecules located at the surface of this region predominate in theattenuation spectrum. This is a particular advantage for obtaining ATRspectra of small samples that must be kept submerged under water, e.g.biological samples. A relatively large pool of aqueous buffer can coverthe surface of the entire waveguide and its supporting quartz substrate.Even when the entire waveguide is covered, this produces only about asmuch background attenuation as is shown in FIG. 10A, i.e. maximally0.2–0.3 absorbance units. Meanwhile, a biological sample that coversonly the ˜1 mm² area above the thinnest portion of the waveguide can bedetected and analyzed with great sensitivity.

The disclosed coupling method enables measurement of ATR-IR spectrausing <100-μm thick planar waveguides in a horizontal configuration. The20 μm thick waveguide affords high attenuation values for a small numberof IR-absorbing molecules at the waveguide surface. This, and theimprovement in signal/noise ratio obtained as a result of the couplingefficiency, make tapered Ge waveguides particularly well suited formeasuring spectra of small biological samples, such as the detection ofdifferent spectra from various components of the cell membranes ofindividual frog eggs, 1.5 mm in diameter, that must be submerged under abulk aqueous buffer.

The quasi-tapered waveguide 200 illustrated in FIG. 11 is tapered as setforth above. The arcuate surface of the Ge prism 202 is then coated witha ZnS coating 204 and embedded into an epoxide substrate 208.

In FIG. 12 the graphed spectra illustrates the comparison betweenScotch® Tape, rubber cement and acetone. The spectra were read using thewaveguide 200 having a 12 μm waveguide nadir. As can be seen in thegraph, the Scotch® Tape 300 and the rubber cement 302 have similarspectra, showing that the tape is invisible and that the only materialreadable is the adhesive. The acetone spectrum 304, however, provides acompletely different spectrum reading than the two adhesives.

In FIG. 13 the absorbance spectrum of D2O, using the waveguidearrangement of FIG. 11, is compared at different waveguide thickness. Asillustrated, the sensitivity of the waveguide increased dramaticallywhen using a 12 μm waveguide. The overall sensitivity increase issubstantially greater than the increase between the 70 μm and 30 μmreadings. FIG. 14 illustrates the spectra of halorhodopsin using the 12μm waveguide of FIG. 11.

The disclosed planar slab waveguides are the thinnest to date capable ofevanescent-wave sensing in the mid-IR. When coupled to an IR microscope,these evanescent-wave sensors show a substantial improvement in surfacesensitivity over thicker waveguides and fibers. These include graduallybi-tapering the waveguide by a factor of 4 or more in both its width andthickness. This will permit an even larger fraction of the guided lightenergy to be propagated as an evanescent wave at the waveguide'sthinnest region, where is where the sensing of microscopic samplesshould take place. Tapering in this manner, rather than uniformlyreducing the waveguide thickness, is a means of allowing more efficientcoupling of light by the IR microscope into and out of all of theallowed modes of the thinnest region of the waveguide. A finer opticalpolish of the Ge surfaces will also enhance the detectivity byincreasing the throughput.

Although the foregoing relates to measuring IR absorption spectra withbroadband light, the waveguide design is also useful for making sensorsbased on monochromatic (e.g. laser) light. These sensors are useful forthe study of very small samples, such as the membranes of single livingcells.

1. A method of fabricating a quasi-planar waveguide, comprising thesteps of: a- cementing a first surface of a polished IR transparentmember to a substrate, b- grinding a second surface of saidIR-transparent member to form an arcuate surface having apexes atopposing ends and a nadir in between, c- polishing said second surfaceto a predetermined thickness.
 2. The method of claim 1 furthercomprising the step of polishing said second surface to have a thicknessof less than 100 micrometers.
 3. The method of claim 1 furthercomprising the step of polishing said arcuate surface to have a ratio ofnadir to apex at opposing ends of up to about 0.25 to
 1. 4. The methodof claim 1 further comprising the step of polishing said arcuate surfaceto have a ratio of nadir to apex at opposing ends of up to about 0.02to
 1. 5. The method of claim 1, wherein said IR-transparent member isgermanium.
 6. The method of claim 1, further comprising the step ofcoating a surface of said IR-transparent member with a cladding, saidcladding being a chemically vapor deposited layer of an IR-transparentmaterial, said layer being from about 1 to about 5 microns in thicknessand having a lower refractive index that of said IR-transparent member,said step of coating a surface of said IR-transparent member with acladding, preceding said step of cementing said IR-transparent member toa substrate, said cladding being between said IR-transparent member andsaid substrate.
 7. The method of claim 5, wherein said step of claddingcomprises chemically vapor depositing a member selected from the groupconsisting of ZnS and ZnSe on said IR-transparent material.
 8. Themethod of claim 1 further comprising the steps of: a. grinding saidarcuate surface to proximate said nadir, b. minimizing light scatteringby polishing said first surface to about a 0.1 μm finish, c. couplingdirectly to an IR detector, d. passing light from a light source,through said waveguide, to said IR detector, wherein said ratio betweensaid apex and said nadir increases IR signal throughput.
 9. A method offabricating a quasi-planar waveguide, comprising the steps of: a-coating a first surface of an IR-transparent member with a cladding,said cladding being from about 1 to about 5 microns in thickness andhaving a lower refractive index that of said IR-transparent member, b-cementing said first surface of a polished IR transparent member to asubstrate, c- grinding a second surface to a predetermined thickness toform an arcuate surface having apexes at opposing ends and a nadir inbetween e- polishing said second surface of said IR-transparent memberto have a ratio of nadir to apex at said opposing ends of up to about0.25 to 1, to about 0.02 to 1, f- coupling said waveguide to an IRdetector; g- passing light from a light source, through said waveguide,to said IR detector.
 10. The method of claim 9, wherein saidIR-transparent member is germanium.
 11. The method of claim 9 claimwherein said cladding is a chemically vapor deposited layer of anIR-transparent material.
 12. The method of claim 9, wherein said step ofcladding comprises chemically vapor depositing a member selected fromthe group consisting of ZnS and ZnSe on said IR-transparent material.